Inside GNSS Media & Research

MAR-APR 2018

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50 Inside GNSS M A R C H / A P R I L 2 0 1 8 e desktop tools run on Windows, Linux and Mac OS. e RF column ( Figure 1 ) shows: a) e strongest four satellites from each constellation b) e time plot of C/N 0 of all satellites c) e skyplot of satellite positions e Clocks column shows: a) e pseudoranges b) e offset frequency of the receiver clock. is is com- puted using a reference position — either: i) Automatically computed mean position ii) User entered lat,lon,alt iii) NMEA file with truth reference PVT is is the first major benefit of raw measurements: you can see the receiver clock behavior to better than 1ppb (part per billion) precision. is is a really important thing to see when you are making a phone — because any heat source near the reference oscil- lator inside the phone may cause the clock error to ramp rapidly. And the best way to see this, in the actual phone, is to use the raw measurements like this. c) e offset of the standby clock that keeps time if the receiver duty cycles the primary oscillator e Measurements column shows: a) The weighted least squares (WLS) position results obtained from the raw and smoothed pseudoranges b) The residual errors of each pseudorange, raw and smoothed c) e residual errors of each pseudorange-rate (-Doppler) measurement is is the second major benefit of raw measurements: you can see the errors of each measurement, and this allows signifi- cant insight into the signal environment and receiver behavior. Raw Measurements and Derived Data e Android API describes truly raw measurements. One of the first things you might notice when you examine the API (see Table 1 ) is that there are no pseudorange measurements. is is because pseudorange is not a raw measurement, it is derived from received satellite time (ReceivedSvTimeNanos). is is something of a paradigm shi for readers brought up on survey receivers that output pseudoranges. But remember that the primary purpose of the GNSS Raw Measurements API is to observe (and thus improve) the operation of the GNSS receiver, and that is why the APIs describe the fundamental raw values. It is our hope and intent that developers will create apps that build on these measurements, providing many derivations of the raw measurements. Indeed this has already started with apps to do PPP and generate RINEX data from your phone (see Additional Resources at the end of this article). How do you get pseudoranges from these values? e anal- ysis tools will do it for you, and if you want to do it yourself, FIGURE 1 The desktop tools provide interactive plots, organized into three columns showing behavior of: RF, Clock, and Measurements. ANALYSIS TOOLS

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